In the autumn of 1944 von Wimmersperg designed two types of simplified assault rifles chambered for the Polte short cartridge a long version and a short version-for which the drawings are held at the BWB/WTS in Koblenz. The captions identify the long version as the Spz-l, and the short version as the Spz-kr. The "Spz" is apparently an abbreviation of the code name Spielzeug (toy). The "1" stands for lange Bauart (long type), while "kr" stands for kurze Bauart mit Regler für Serienjeuer (short type with regulator for sustained fire).

Both the rifle and the compact carbines consist mainly of stamped parts, many recognisable as having been directly or indirectly derived from the Sten MkII MP. Both utilise the standard StG44 magazine and magazine catch, and StG44 barrel blanks. The receiver tubes are original Sten parts with a diameter of 35mm and a wall thickness of 2mm. The rear socket section of the two-piece turning bolt is a modified Sten component, and the operating handle, recoil spring, its housing and end cap assembly, with its method of affixing the butt stock, are standard Sten parts as specified in the relevant parts lists.

The gas port is positioned at the forward end of the barrel about 21cm (8.25") back from the muzzle. The gas piston, located on the right side of the barrel on a 60° diagonal to the vertical, led into a gas chamber inside the barrel jacket. The gas block enclosing the barrel held the gas regulator, configured as a spring-loaded pin by which the vent could be regulated or closed by means of the lever on top of' the gas block. Both short and long types have remov¬able barrels, like the MP40. The rear end of the barrel is held by a barrel tube, which fitted into the fore end of the receiver tube and was held in position by the barrel nut. The rear part of the barrel tube served as a locking piece for the two-piece turning bolt, while the gas piston acted on the bolt head through cutouts in the barrel tube.

The Spz-1 and Spz-kr were hammer-fired, while the SPz-kv (kv standing for kurze Bauart mit Vcrshlußzündung; short type with bolt ignition; fig 160) fired from the closed bolt, actuated by the bolt socket. All were capable of single shot and sustained fire. Single-shot fire could be applied by pulling the trigger half way, while pulling the trigger further to the rear produced burst fire.

The firing system housing, some of the trigger components and the butt stock of the long version are very reminiscent of the MKII Sten, and the sights and general layout are conventional. Not so the com¬pact carbine, whose action has been truncated into a "bullpup" configuration with a curved metal butt plate affixed directly to the end of the receiver tube (fig 159). A firing mechanism housing is fixed onto the underside of the receiver cylinder, with the trig¬ger guard at its forward end and the magazine well following the rear part of the trigger guard, where an inserted StG44 magazine served as the pistol grip.

The sight axis on the carbine was about 50mm above the upper surface of the cylinder, whereon an appropriately located rear sight base of sheet metal was welded, slightly to the left of the longitudinal axis of the receiver. The upper part of the rear sight's side walls were provided at the rear with four vertically spaced rectangular steps, each representing 100m, so that the rear sight leaf could be raised or lowered. presumably by means of a sliding bar, to cover ranges from 100 to 400 metres. A metal dovetail bracket to accept the regular ZF4 scope mount was welded onto the right side of the carbine receiver, so as not to obstruct the rear iron sight. A rectangular front sight base, provided with a hood to protect the front sight blade, was welded onto the plain muzzle end of the barrel. The front sight blade on the Spz-1 version is a standard K98k part.

Two different firing mechanisms were proposed for the Spz-kv carbine, one with a more-or-less conventional hammer as used in the Spz-1 (fig 1581 and the later variant, produced in January, 1945. fitted with a spring-loaded rotating disk-like regulator with delay detents controlling a more forwardly positioned hammer.

No official documents dealing with the von Wimmersperg assault rifle exist, other than the drawings he made and signed himself It is quite possible that he made up these drawings for use by an armsmaking firm, as before the war he had been in negotiation with firms such as Mauser, Simson & Co, Fokker, etc, concerning the actual construction of his early self-loading rifle and machine gun prototypes.